The invention relates to an apparatus and a method for monitoring the autonomous nervous system of an individual, especially for detecting pain.
The invention also relates to a method for controlling a warning signal in an apparatus for monitoring the autonomous nervous system of an individual, especially for detecting pain.
In the field of medical technology there is a problem in producing physical measurements representing the activity in an individual""s autonomous nervous system, i.e. in the part of the nervous system which is beyond the control of the will. It is particularly important to obtain indications of the activity in the sympathetic nervous system. For example, in various situations there is a need to monitor a patient""s experience of pain.
There is a special need to monitor the sympathetic nervous system in babies. It is known that babies with apnea (a cessation of breathing for more than 20 seconds) and attacks of lifelessness exhibit changes in the degree of activity in the sympathetic nervous system. The indications are, therefore, that monitoring the sympathetic nervous system of babies may contribute towards revealing dysfunctions in the nervous system, and that such monitoring can also be used to warn of the risk of cot death.
A method and an apparatus for monitoring the autonomous nervous system in an individual will also be applicable in other situations. In the case of both premature babies, infants and other individuals, both children and adults, there may be a need to observe pain reactions. This applies in particular to cases where the individual himself is unable to express the experience of pain in the normal manner, for example verbally, by means of crying or facial expression.
An example of this kind of application is monitoring of premature babies. These babies have a special need for pain monitoring, since they have poorly developed facial expression, in addition to which they may lack the energy to cry. In premature babies pain or stress have been associated with the occurrence of cerebral haemorrhage, and it has also been shown that their experience of pain can be remembered, thereby affecting their future pain reactions. It is therefore vital to know in real time when such babies are exposed to pain, in order, amongst other things, to be able to administer analgesics.
Another application is monitoring of patients in a respirator.
The sympathetic nervous system can be activated by the feeling of pain, but also by other factors such as stress, fear and anger. When the sympathetic nervous system is activated, it causes reactions such as increased heart rate, increased blood pressure and increased emotional perspiration. Blood pressure, pulse and respiration rate are controlled both by the sympathetic and parasympathetic nervous system, as well as being affected by other factors, such as if the patient loses a lot of blood, or has lung or heart disease. Of all these phenomena, therefore, increased emotional perspiration is the most specific target for the activity, particularly for detecting pain response in the sympathetic nervous system.
A known phenomenon associated with emotional perspiration is that the skin""s conductance, particularly on specific parts of the body such as in the palms of the hands and the soles of the feet, is influenced by the activity in the sympathetic nervous system, caused among other things by stimulation of the sense of pain. On exposure to pain stimuli or other stress the sympathetic activity in efferent nerve fibres to the sweat glands increases. When the sympathetic nerves are activated, the sweat glands are activated, the sweat channels are filled with liquid, and the conductance in the skin increases. When the liquid evaporates, the skin conductance decreases. In this manner fluctuations arise in the skin conductance. This phenomenon is called spontaneous skin conductance. The spontaneous skin conductance consists of waves and a basal level. The number of waves and the height of the waves indicate direct sympathetic activity in the nerves.
The basal level, however, does not constitute a satisfactory basis for drawing conclusions concerning the activity of the patients sympathetic nervous system, including the presumed pain condition. Amongst other things this is due to the fact that after a pain or other nervous system stimulation, the total skin moisture level and thereby the conductance signal takes a relatively long time to return to its basal level (the so-called recovery time).
However, tests have shown that the skin""s conductance appears as a time variable signal which, in addition to a basal, slowly varying value (the so-called basal level), also has a component consisting of spontaneous waves or fluctuations, in which characteristics of these fluctuations, such as for example their frequency and amplitude, are factors which are correlated with the experience of pain in the target object (the patient). Measuring and analysing characteristics of these fluctuations may therefore be a suitable method of providing fast and reliable information concerning the activity in the sympathetic nervous system, including the effect of pain.
Tests which compare the activity in the sympathetic nervous system with skin conductance, where the frequency of maximum values and the fluctuations in the skin""s conductance are taken into account, are known from, amongst others, Edelberg R: Electrical Properties of the Skin. Methods in psychophysiology (Brown, C. C. (ed.)), Ch. 1, The Williams and Wilkins Company 1967, pp. 1-50, and from Lidberg L., Wallin G. Sympathetic skin nerve discharges in relation to amplitude of skin resistance responses. Psychophysiology 1981;18(3):268-270.
Previously known systems for analysing skin conductance comprise a data acquisition system for recording a series of measurements for the skin""s conductance, and a computer for subsequent analysis of the recorded series of measurements. The analysis therefore takes place after the data have been collected, and thereby after the autonomous nervous activity in the patient, as well as the sensations which gave rise to the nervous activity, have already ceased. Such systems therefore offer no possibility of real time monitoring of the patient""s autonomous nervous activity and pain reaction. In particular, they offer no possibility of detecting and warning that a limit for pain reaction has been exceeded during the period in which the measurements are performed.
From the patent literature several solutions are known which have some points of resemblance with the present invention:
U.S. Pat. No. 5,897,505 discloses a diagnostic apparatus for assessing pain in a human being, based on skin conductance and temperature measurement. The apparatus is intended for performing a single measurement at a time, and to indicate the result of this measurement on a display. The apparatus also includes auto-scaling and overrange functions which implies that two subsequent measurements are performed, in order to determine the appropriate scaling of the input signal and possibly to indicate an overrange warning signal. The apparatus does not permit the continuous monitoring and analysis of spontaneous response in skin conductance, particularly not the amplitude and frequency of fluctuations in the skin conductance signal, caused by pain or similar activities in the autonomous nervous system of the individual.
WO 85/00785 discloses an attention monitor based on skin resistance measurement. A warning signal is activated when the measured resistance exceeds a predetermined amount. The publication does not indicate or suggest the analysis of spontaneous response in skin conductance, caused by pain or similar activities in the autonomous nervous system of the individual.
U.S. Pat. No. 4,697,599 discloses an example of a previously known apparatus for detection of pain, based on measurement of the skin""s conductance. The apparatus exhibits a measurement on a display, in addition to which it emits an audible signal which has a pulse frequency which varies in accordance with the measured conductance. The apparatus only supplies information on the immediate value of the conductance, and does not analyse the measurements with regard to the frequency and amplitude of spontaneous fluctuations. The apparatus moreover uses a direct current-based resistance measurement, which can lead to side effects from the skin""s polarisation properties.
GB-A-2.291.971 describes an example of an apparatus for visual biofeedback therapy. The apparatus is based on the visualisation of an autonomous activity for the patient. This is achieved by a result of the autonomous activity being measured by a measuring device, which in this case may comprise a sensor for measuring the skin""s conductance. The result of the measurement then influences an image which is displayed to the patient. A feedback loop is thereby created which can enable the patient to train himself to reproduce relaxation exercises which alter the measured value in the desired direction. There is no indication that the apparatus comprises equipment for analysing spontaneous fluctuations, including their amplitude and frequency, in the measured conductance signal. Nor is there any indication that the apparatus has an area of application within the field of real time monitoring of a patient""s sympathetic nervous system, including pain monitoring, but it is intended to be applied in the treatment of complaints such as, e.g., irritable stomach/bowel syndrome.
WO 86/01317 describes a method and an apparatus for utilising electrodermal response as a control body for a computer. In this case the skin""s resistance is measured by means of a so-called paddle input body, equipped with electrodes, for a computer. In the method and the apparatus a certain amount of consideration is given to the dynamic of the resistance signal, not only the signal""s immediate value, but also the signal change from one point of time to the next, being used in the calculation of a control signal. However, the publication gives no analysis of the amplitude and frequency of spontaneous signal fluctuations for monitoring purposes, and for detection of pain in particular. In this publication too resistance measurement based on direct current is employed, which may result in the skin""s polarisation properties influencing the measurements.
Related solutions can also be found in the part of the technology which deals with lie detectors. Some lie detectors employ skin conductance as the basis of their analysis. However, such equipment does not make use of secondary characteristics in the skin conductance signal such as the frequency and amplitude of spontaneous fluctuations for real time monitoring of the activity in the sympathetic nervous system, and for detection of pain in particular.
Thus an object of the present invention is to provide an apparatus for monitoring the autonomous nervous system in an individual, especially for detecting pain, which is not encumbered by the disadvantages which are mentioned above.
According to the invention this is achieved by an apparatus as mentioned above, characterized in that it comprises the features which are indicated by the characterising part of the following independent claim 1.
It is a further object of the present invention to provide a method for controlling a warning signal in an apparatus for monitoring the autonomous nervous system in an individual, especially for detecting pain, and which is not encumbered by the said disadvantages.
According to the invention this is achieved by a method as mentioned above, characterized in that it comprises the features which are indicated by the characterising part of the following independent claim 6.
Finally, it is an object of the present invention to provide a method for monitoring the autonomous nervous system in an individual, especially for detecting pain, and which is not encumbered by the said disadvantages.
According to the invention this is achieved by a method as mentioned above, characterized in that it comprises the features which are indicated by the characterising part of the following independent claim 10.
Further advantages and characteristics are indicated in the dependent claims.